Acoustic waves (AW) in piezoelectric structure have been used to measure liquids in contact with the piezoelectric structure. For example, U.S. Pat. No. 4,378,168 of H. Kuisma, Mar. 29, 1983, discloses a piezoelectric device having spaced input and output electrodes on a surface for generating a surface acoustic wave (SAW) that detects the presence of humidity between the electrodes as a function of signal attenuation. However, this device does not provide an indication of any properties of condensed liquid.
The use of devices which use the Rayleigh wave, or SAW, to sense mass changes at solid/gas interfaces is known. However, the SAW is impractical for use in detection at solid/liquid interfaces because the Rayleigh wave they employ does not propagate efficiently at a solid-liquid interface. The Rayleigh wave has a surface-normal component of particle displacement which generates compressional waves in a liquid, leading to substantial attenuation of the wave.
U.S. Pat. No. 4,691,714 of J. Wong et al., Sep. 8 1987, discloses the use of a bulk acoustic wave within a piezoelectric structure to measure viscosity of a liquid in contact with the structure. The viscosity of the liquid is a function of the amplitude of the transmitted bulk acoustic signal, and the temperature of the liquid (actually, the temperature of the sensing transducer) is a function of the phase of the SAW that is also transmitted in this device.